This invention relates generally to a web feed apparatus of the type set forth in commonly owned U.S. Pat. No. 3,955,737, and more particularly to such an apparatus having a feedback function between low and high web tension.
The aforementioned commonly owned patent is specifically incorporated herein by reference. The patent discloses a nest of snubbing or capstan rolls located between a higher web tension section of web operation and a lower web tension section at web feeding and metering, for the purpose of isolating the web tensions from each other so that the desired metering accuracy at the lower web tension level at the web infeed and metering means is maintained. This isolation or force multiplication, while effective, is nevertheless limited by uncertainty as to the value of friction which can be depended upon in the capstan roll nest. Hence, the total angle of wrap of the web about the snubbing rolls must be small enough so that a tight web may be maintained at the highest possible friction value. This in turn limits the highest tension at the web operational section because the low tension side of the snubbing roll nest must be limited to a particular value. In other words, because of the uncertainty of tension ratio caused by variation in the coefficient of friction, the total wrap angle must be limited and hence the maximum tension on the high tension side of the nest must also be limited.
For example, the effect of the snubbing rolls is to isolate the high web tension from the low web tension in accordance with the basic capstan equation T.sub.out /T.sub.in = e.sup..mu..beta., wherein T.sub.out is the high side web tension in pounds per inch, T.sub.in is the low side web tension in pounds per inch, .beta. is the total angle of capstan wrap in radians, and .mu. is the apparent coefficient of friction between the web and the snubbing rolls (capstan surface). If the low side tension is limited to 0.05.ltoreq.T.sub.in .ltoreq.0.15, and the coefficient of friction is limited to 0.2.ltoreq..mu..ltoreq.0.3, and the high side tension is limited to T.sub.out .gtoreq.1.0, for the highest friction coefficient 1/0.05 .ltoreq.e.sup.0.3.beta., then .beta..ltoreq.572.degree. or 9.98 radians. Therefore, .beta. cannot be any larger than 9.98 radians. On the other hand, if the friction coefficient decreases to 0.2 the value of web tension at the high side will be limited as follows because of the maximum allowable value of T.sub.in = 0.15; T.sub.out /0.15 = e.sup.0.2(9.98), then T.sub.out = 1.1 pounds/inch. Therefore, because of the uncertainty of the sliding coefficient of friction the tension range of T.sub.out is limited to 1.1 minus 1 or 0.1 pounds per inch. It is therefore desirable to increase this range and to maintain T.sub.in nearly constant over a wider range of output tensions (T.sub.out).
If the capstan nest in accordance with the principles set forth in the aforementioned U.S. Pat. No. 3,955,737 is likened to an electronic or mechanical amplifier, it would be considered to be operating open loop. That is, the value of the input web tension (T.sub.in) is not capable of being held approximately constant by modifying the overall gain of the capstan nest, in response to the value of T.sub.in.